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Northwest Geology Field Trips, by Dave Tucker, is licensed under a Creative Commons Attribution-Noncommercial- Share Alike 3.0 United States License. You can use what you find here, repost it with attribution to the author, "remix" it for your own purposes, but may not use it with the intent of making money off of it.

EDUCATORS: Please feel free to use anything you find here that is useful to your mission educating people about Earth science. E-mail me if it would help to have a larger or higher-resolution version of any of the images. tuckerd at geol dot wwu dot edu

View to west. The Capricorn debris flow descended Mount Meager’s east flank into Meager Creek, jogged north slightly, then spilled down Lillooet River (foreground). BCFS campground is visible as the vegetated area left of and adjacent to the roadcut at lower right. Photo T. Spurgeon. Click to enlarge any photo.

In the early morning hours of August 6, 2010 a very large debris flow descended the valley of Capricorn Creek, crashing into the opposite valley wall as it deflected northward in the Meager Creek drainage, then flowed east upon joining the Lillooet River. The flow originated on the upper slopes of the Mount Meager volcanic complex, 66 km (41 miles) northwest of Whistler, BC. The debris flow scoured a path that destroyed everything it encountered on its way to joining with the Lillooet River. It took out the road leading to the Meager Hot Springs (a well-known recreation site, posted with signs announcing debris flow risks), pulverized a well-engineered new bridge that crossed the Capricorn Creek debris flow channel, and created several depositional dams, large ponds and several lakes that for a time threatened even more destruction from outburst floods. Approximately 40 million cubic metres of debris were discharged in what is one of Canada’s largest recorded debris flows.

View to east looking down Capricorn Creek. Source area in extreme upper left. T. Spurgeon photo.

The headwaters of Capricorn Creek are on Mount Meager, a volcano active in the Holocene (Hickson and others, 1999). A large explosive eruption occurred 2360 14C years BP at Plinth Peak, a subsidiary vent on Meager’s north flank, and sent pyroclastic flows down the Lillooet valley. A map showing the location of Meager relative to the rest of BC’s Garibaldi Belt is here. The confluence of Capricorn and Meager Creeks is N50º36.286′, W123º25.800′. Zoom in on the latest Google Earth image at these coordinates to see the fresh debris flow, as well as the 340-meter-long lake impounded behind it.

A large explosive eruption occurred 2360 14C years BP at Plinth Peak, a subsidiary vent on Meager’s north flank, and sent pyroclastic flows down the Lillooet valley. The confluence of Capricorn and Meager Creeks is N50º36.286′, W123º25.800′. Zoom in on the latest Google Earth image at these coordinates to see the fresh debris flow, as well as the 340-meter-long lake impounded behind it.

Take a trip to see the devastation left behind by the September debris flow, an example of Mother Nature’s version of “shock and awe”. Even the huge body of aerial imagery on the Internet doesn’t do it justice. See it from the ground! So, here is my report of a day trip from Vancouver (September 25, 2010) to see a favourite area where I have spent a lot of time over the years. This is not intended to be anything like a serious geological report; it is mainly a guide to clarify area access, indicate some things to see, and state some common-sense cautions.

Travel north from Vancouver 154 km (97 mi) along the scenic Sea to Sky Highway (BC 99, all paved) through Squamish and Whistler to Pemberton (about 2.5 hours); Pemberton is 174 km (108 mi) north of the US border at Blaine, Washington. Get snacks and gas up in Pemberton– I never pass by a gas station when heading into this sort of wilderness.

Turn left off 99 into Pemberton. Set odometer to 0.0. Follow the signs north out of town; exiting 99, you’ll begin on Portage Road, go around a traffic circle onto Birch, intersect with Prospect, and then drive through the agricultural Pemberton Valley. Prospect intersects with Pemberton Meadows Road– turn left here.

24.4 km (14.6 mi) from Highway 99, turn right (north) on the Lillooet Forest Service Road (FSR). From fields a short distance up this road, you may be able to see up the valley to the Meager volcanic complex in the distance.

25.9 km (15.5 mi) Cross the Lillooet River; pavement ends. This is serious backcountry. 2 WD works fine on these roads. SUVs do this road easily so no problem until something goes wrong. It is an active logging and mining haul road so give way to the trucks, they always win!

33.2 km (19.9 mi) Important junction! Veer left on the BCFS road (signed “to Meager Creek Hot Springs”). Do not go right uphill onto Hurley Road 60.4 km (36.2 mi) BC Forest Service campground, on your left. The field trip is in the vicinity of this campground.

A warning sign was located just across the washed-out Lillooet River bridge on the closed Meager Creek Road in 2008. Apparently destroyed in 2010 or maybe underwater. T. Spurgeon photo.

A word about road signs is in order. We saw plenty of traffic along the FSR: looky-loos, mushroom pickers, firewood gatherers, hikers, ATVers, researchers, etc. The signage varies from dire consequences for inattentive travelers to more reasonable warnings and directions. It seems that signs get put up but hardly ever taken down. Sure, the signs say Meager Creek Hot Springs Road is closed – no kidding, after all it and the bridges are gone! This is not applicable to the Lillooet FSR though, since it is in decent condition. The two areas along it that were obliterated by the debris flow have been cleared, repaired, and are in good shape. Most of the accessible good stuff to see is in the vicinity of the BC Forest Service campground, which was untouched, albeit it was initially cut off. This is a good place to camp if you overnight (gravel pads, fire pits, toilets) and gives good access to the debris flow deposits in the Lillooet River floodplain. Mind you I would be uncomfortable staying there if it was raining heavily as this is still a pretty dynamic environment. Take good boots and raingear along. Black and grizzly bears frequent the area; carry bear spray and know how and when to use it; otherwise you’ll just season yourself and add spice for the bears. Here’s a topographic map showing extent of the landslide. Cameras and note taking are mandatory!

Area debris flows

Numerous debris flows have occurred in the past, (see photo of the sign), all initiating on the upper flanks of the Meager volcanic complex. Another drainage on the south flank, aptly known as Devastator Creek, was the site of a huge landslide and debris flow on July 22, 1975. Four BC Hydro geologists exploring the geothermal potential of the area were killed in that event. A previous large event occurred in Capricorn Creek in 1998. There is a very readable paper published online describing the 1998 event (Bovis and Jakob, 2000). The paper explains the reasons behind these destructive flows and the resulting deposits. The paper could just as well be applied to the September 2010 debris avalanche and flow. Essentially, flows are initiated in unconsolidated glacial and volcanic colluvium high on the Meager edifice. Continued glacial retreat removes support for oversteepened slopes, which can then fail in heavy rainfall or in periods of high temperatures.

Near the campground, head out onto the hummocky debris field (this is essentially the last 500 metres or so of the bulked-up flow). I noted clay balls or agglutinated masses of the stickiest clay I have ever touched. The clayey balls are sub-rounded, matrix-supported masses containing maybe 50% clasts. Clasts range from rice grain-sized granules to tangerine-sized cobbles, some angular, some rounded. The clay masses at the time I saw them are

Also, watch for high water mud marks on trees, debris along the Lillooet River channel, and note size changes in the cobbles on the river bars as you glimpse the river channel while driving north. Take care on the verges where the heavy machines cleared the road as the debris there is wet, uncompacted and take it from me, veeerrry mushy. The flow debris is generally very compact. Access out onto the floodplain is easy from the north end of the BCFS campsite area, along a new but hard to see cat-track. Along the road you can also access the hummocky deposits where geologists have spray painted or marked foot routes off the road. Park only in wide areas – those mining and log hauling trucks are BIG! Do note the fresh cut bank on the north end of the BCFS campsite where a nice cross-section of previous flow deposits is revealed.

If you continue upstream beyond Meager Creek in the Lillooet valley is possible to visit a world class volcanic environment featuring a host of different volcanic deposits that will whet your curiosity about the eruption(s) at Meager. Watch this website for a field trip to that area. In the meantime, a field trip to the Meager/Plinth Peak eruption deposits is published in Russell and others, 2007. See the references.

If you choose to make this an overnight, there should be plenty of time to take in the new Mining Museum at Britannia, along Howe Sound between Horseshoe Bay and Squamish – it is well worth the stop.

More on the web

A CTV news report with video with incessant gesticulation during the on-site report [following the investment banking ad; be patient]. There is impressive debris flow footage [digitage?] 35 seconds into the video. The news clip is followed by a separate 45-second video shot from the news helicopter; it is pretty blurry.

Russell, J.K., Hickson, C.J., and Andrews, G., 2007, Canadian Cascade volcanism: Subglacial to explosive eruptions along the Sea to Sky Corridor, British Columbia, in Stelling and Tucker (editors), Floods, Faults, and Fires: Geological Field Trips in Washington State and Southwest British Columbia: Geological Society of America Field Guide 9, p. 1-29, doi: 10.1130/2007.fld009(01).